The present invention relates to an automotive brake control system with an anti-skid braking device (ABS unit) which acts to prevent a wheel lock-up condition during braking and to provide maximum effective braking, and particularly to such an anti-skid braking system (ABS system) capable of variably controlling a rate of pressure build-up depending on the changes in road conditions, that is, a friction coefficient of the road surface.
ABS systems usually employ rotational wheel speed sensors that monitor each wheel""s rotational speed and send a signal back to an on-board computer to indicate a deceleration rate of each wheel. If the wheel speed sensor signals indicate that a wheel or wheels are locking up, the ABS device comes into operation to momentarily reduce a wheel-brake cylinder pressure of the wheel subjected to skid control and thereafter to again build up the wheel-cylinder pressure when the wheel speed recovers near to the vehicle speed and thus there is no risk of the wheels to lock up. In this manner, the ABS system operates to prevent skidding and to shorten a braking distance as much as possible, and thus to provide maximum effective braking during anti-skid operation. In recent years, there have been proposed and developed various ABS systems in which a rate of pressure build-up following the pressure-reduction operating mode (or reduce-pressure mode) can be varied depending upon road surface conditions, during skid control. In such ABS systems, the rate of pressure build-up is determined or set depending on the road-surface friction coefficient (hereinafter is referred to as a xe2x80x9croad-surface xcexcxe2x80x9d), and thus it is possible to provide the shortest possible braking distance without incurring wheel lock-up and vehicle skidding, during braking on both a high-xcexc road surface condition (e.g., dry pavement) and a low-xcexc road surface condition (e.g., snow or icy roads). For instance, during braking on the high-xcexc road, it is required to set the rate of pressure build-up following the reduce-pressure mode at a comparatively great value, so as to properly reduce a recovery time that the wheel-brake cylinder pressure recovers near to a suitable brake fluid pressure, and thus to shorten the stopping distance. Conversely, during braking on the low-xcexc road, it is required to set the rate of pressure build-up at a comparatively small value, so as to prevent the wheel from again starting to lock due to an improperly high pressure build-up rate during one cycle of the pressure build-up mode. Skid control based on the relatively great pressure build-up rate programmed to be suitable for the high-xcexc road surface condition is often called as a high-xcexc control mode (simply, high-xcexc control), whereas skid control based on the relatively small pressure build-up rate programmed to be suitable for the low-xcexc road surface condition is often called as a low-xcexc control mode (simply, low-xcexc control). On a moment""s thought, the road surface conditions are not constant. That is, during vehicle driving, low-xcexc roads are usually sprinkled or dispersed. Therefore, there is an increased tendency for the road surface condition to change from the low-xcexc road surface condition to the high-xcexc road surface condition.
It is an object of the invention to provide an automotive brake control system with an anti-skid braking device, which is capable of accurately detecting a difference between low-xcexc road and high-xcexc a road.
In order to accomplish the object of the present invention, an automotive brake control system comprises vehicle sensors which detect at least each wheel speed of road wheels to generate wheel speed sensor signals, an anti-brake skid unit which controls a wheel-brake cylinder pressure to each individual wheel cylinder of the road wheels to prevent a wheel lock-up condition during braking, a control unit being configured to be connected electrically to the vehicle sensors and the anti-brake skid unit to execute skid control having at least a reduce-pressure operating mode and a pressure build-up operating mode, when the wheel speed sensor signals indicate that at least one of the road wheels is locking up, the control unit comprising a road-surface-condition change determination section which determines, based on both a time length of brake-fluid-pressure control continuously executed during the skid control and a recovery acceleration of the wheel speed of the road wheel subjected to the skid control, whether there is a change of a road surface condition from a road surface having a low friction coefficient to a road surface having a high friction coefficient.
According to another aspect of the invention, an automotive brake control system comprises vehicle sensors for detecting at least each wheel speed of road wheels to generate wheel speed sensor signals, an anti-brake skid unit for controlling a wheel-brake cylinder pressure to each individual wheel cylinder of the road wheels to prevent a wheel lock-up condition during braking, a control unit being configured to be connected electrically to the vehicle sensors and the anti-brake skid unit to execute skid control having at least a reduce-pressure operating mode and a pressure build-up operating mode, when the wheel speed sensor signals indicate that at least one of the road wheels is locking up, the control unit comprising a road-surface-condition change determination means for determining, based on both a time length of brake-fluid-pressure control continuously executed during the skid control and a recovery acceleration of the wheel speed of the road wheel subjected to the skid control, whether there is a change of a road surface condition from a road surface having a low friction coefficient to a road surface having a high friction coefficient.
According to a still further aspect of the invention, an automotive brake control system comprises vehicle sensors which detect at least each wheel speed of road wheels to generate wheel speed sensor signals, an anti-brake skid unit which controls a wheel-brake cylinder pressure to each individual wheel cylinder of the road wheels to prevent a wheel lock-up condition during braking, a control unit being configured to be connected electrically to the vehicle sensors and the anti-brake skid unit to execute skid control having at least a reduce-pressure operating mode and a pressure build-up operating mode, when the wheel speed sensor signals indicate that at least one of the road wheels is locking up, the control unit comprising a road-surface-condition change determination section which determines, based on both a time length of brake-fluid-pressure control continuously executed during the skid control and a recovery acceleration of the wheel speed of the road wheel subjected to the skid control, whether there is a change of a road surface condition from a road surface having a low friction coefficient to a road surface having a high friction coefficient, the road-surface-condition change determination section being provided individually for at least left and right wheels of the road wheels.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.